(197bp) Computational and Experimental Studies of Selexol Solvent Physical Properties and Its Interactions with H2o, H2, and CO2

Selexol is a physical solvent, which can be used to selectively remove acid gases, such as CO₂ and H₂S from a variety of natural and synthesis gas streams ^[1,2]. Selexol is very hydrophilic, which could absorb water from the gas stream and as a result to unfavorably decrease its performance in CO₂ capture ^[3]. In order to conduct reliable process modeling to analyze CO₂ capture in H₂ production using the Selexol solvent compared with hydrophobic solvents (such as diethyl sebacate ^[3]), thermophysical properties of Selexol and its interactions with other gas components, such as H₂O, CO₂, and H₂ are needed.

In this work, we studied various physical properties both from molecular simulations and experiments, such as gas absorption Henry’s law constants and diffusivities for H₂O, CO₂, and H₂ in Selexol at 298-373 K, excess molar volume for solvent mixture, viscosities, and vapor pressures. Both simulations and the experimental data show that H₂O-Selexol solvent mixture exhibits the largest negative excess molar volume of 1.5-2.5 cm³/mol at 298 K and a water mole fraction of 0.83. At this specific water mole fraction, large clusters formed between water and Selexol molecules were observed from simulations, which could maximize water-Selexol interaction and leads to the largest negative excess molar volume. In addition, At about this water concentration, both simulations and experimental data show that the water-Selexol mixture exhibit higher viscosity than at other water concentrations. Simulations show that CO₂/H₂ solubility selectivity in Selexol decreases from 88 ± 1 at 298 K to 39.2 ± 0.3 at 373 K, which are due to decreased CO₂ solubility however increase H₂ solubility at elevated temperatures. In the Henry’s law constant region, both simulations and experimental data show that H₂O/CO₂ solubility selectivity is between 100-10000, indicating that Selexol is indeed very hydrophilic. At 298-373 K, simulations show that gas diffuses in the following relative order: H₂ (100) > CO₂ (10) > H₂O (1). Although Selexol and diethyl sebacate exhibit similar viscosities, both simulations and experimental data suggest that CO₂ diffusivities in diethyl sebacate is 4-5 times smaller than in Selexol. The vapor pressure for Selexol (a mixture of different chains of (CH₂CH₂O)_n) obtained from solvation free energy calculation was computed to be 0.02 ± 0.01 Pa at 298 K, comparable to the literature experimental value of 0.097 Pa. These findings suggest that Selexol is very non-volatile. In addition, at 298 K, the lightest component (CAS 112-49-2) is the predominant component in the gas phase; other heavier components have negligibly small concentrations in the gas phase. When the temperature is increased to 373 K, the saturation vapor pressure was estimated to be 22 ± 3 Pa, with a partial pressure of 19 Pa for CAS 112-49-2, and 3 Pa for the heavier CAS 143-24-8, respectively. Finally, calculations show that the solvation free energies for different components in Selexol correlate linearly very well with the number of repeat units for (CH₂CH₂O).

References:

1. Ashkanani, H. E.; Wang, R.; Shi, W.; Siefert, N. S.; Thompson, R. L.; Smith, K.; Steckel, J. A.; Gamwo, I. K.; Hopkinson, D.; Resnik, K.; Morsi, B. I. Levelized Cost of CO₂ Captured Using Five Physical Solvents in Pre-combustion Applications. J. Greenh. Gas Control 2020, 10, 103135.
2. Burr, B.; Lyddon, L. Which Physical Solvent is Best for Acid Gas Removal? Process. 2009, 88, 43–50.
3. Wei Shi, Surya P. Tiwari, Robert L. Thompson, Jeffrey T. Culp, Lei Hong, David P. Hopkinson, Kathryn Smith, Kevin Resnik, Janice A. Steckel, and Nicholas S. Siefert, Computational Screening of Physical Solvents for CO₂ Pre-combustion Capture, Phys. Chem. B. 2021, 125, 13467–13481.